See here for adding/importing books to Aldiko (if the books still have DRM make sure to register Aldiko to the same Adobe ID before importing)...
https://aldiko.zendesk.com/entries/3...ko-Book-Reader
The only way I know of to get Mantano would be to buy & install it from the Amazon Appstore on another Android device and then do a backup of the app (Amazon generally marks other reading apps as incompatible for the Fire in their store). This will result in an .apk file that can be sideloaded. We discussed it here...
https://www.mobileread.com/forums/sho...d.php?t=238537